Is there a voltage-dependent Cl conductance or do Cl ions modify other conductances in crab muscle fibre?

1. The electrical activity of crab muscle fibre disappeared in Cl-deficient solutions when Cl was substituted by an impermeant anion. 2. Under voltage-clamp conditions, the above solutions abolished Ca-inward current and altered K-outward currents. 3. In 84 mEq-Cl solutions, (Cl substituted by methane sulfonate or proprionate) it is chiefly in gCa-availability that was modified. Half-availability was shifted by 50 to 70 mV in a hyperpolarizing direction while the slope factor was noticeably increased. Similar effects, but of less amplitude, were observed when decreasing the external Cl concentration to 255 mEq. 4. Ca- and slow K-activation variables were shifted by 17 mV and 20 mV respectively in a hyperpolarizing direction by the Cl-poor (84 mEq) solution. Cl-deficiency was even more efficient on the fast outward current than on the Ca current; the former current could be significantly reprimed only if the Cl-concentration was reduced by about 10 per cent. 5. In Cl-deficient solution, electrical activity could be restored by injecting hyperpolarizing currents in order to reprime the Ca-conductance. 6. It is suggested that Cl-ions bind to positive charges on both sides of the membrane. When substituted by impermeant anions, the local anion concentration decreases at the internal surface of the membrane, thus reducing the strength of the effective field of the membrane.